For the seared salmon, season the salmon fillet on both sides with salt and freshly ground black pepper. Melt the butter in a frying pan until frothing, then add the salmon skin-side down and fry for 2-3 minutes, or until golden-brown. Turn the fish over, squeeze over the lemon juice, remove from the heat and allow the salmon to finish cooking in the residual heat.
For the sweet-and-sour tomatoes, place the tomatoes, sugar and red wine vinegar into a saucepan and season with salt and freshly ground black pepper. Gently simmer over a medium heat for ten minutes, or until the tomatoes have softened.
For the wilted watercress, heat a dash of olive oil in a pan, add the watercress and season with salt and freshly ground black pepper. Cook for 1-2 minutes, stirring frequently, until the watercress has wilted.
To serve, pile the wilted watercress onto a serving plate, top with the salmon fillet and spoon the sweet-and-sour tomatoes on the side.